Little is known about the function of the non-collagenous proteins in the organic matrix of bone. Information about these proteins was obtained from studies of human cortical bone. Analysis of fractions of bone differing in density showed that 96% of the α₂HS-glycoprotein and 58% of the albumin was associated with the mineralized phase. A study of age-related changes in the composition of bone showed that the amounts present of α₂HS-glycoprotein, albumin, sialoprotein, soluble collagen and of EDTA-soluble protein were all higher in bone from children than in adults. In subjects aged over 60 y the content in bone of albumin, sialoprotein and of total protein was similar to that of children. These differences could not be correlated with age-related changes in the calcium/hydroxyproline ratio nor with previous reports of alterations in the crystal structure and the physical properties of human cortical bone.